Rechargeable batteries are increasingly used for a variety of energy storage applications. While, lithium ion (Li-ion) batteries remain a very important commercial and research focus, there is an increasing need for new battery technologies to provide better cycling and less capacity loss than the Li-ion.
With regard to the growing environmental crisis resulting from rapid worldwide energy consumption, energy harvested from sustainable renewable sources like solar, wind, or tide is desirable. However, the lack of efficient and economical energy storage devices is still a bottle neck for the practical application of these clean energies. Albeit the great success of lithium-ion batteries (LIBs) in the field of portable electronic applications, the cost and safety barriers make the state-of-the-art LIBs not suitable for large power storage or transmission.
One promising approach to an alternative to Li-ion batteries is a battery based on a multivalent ion electrolyte, such as magnesium (Mg) or calcium (Ca). Li-ion, with a charge of +1, can provide only a single electron for an electrical current, whereas multivalent ions (e.g., with a charge of +2 or +3) have the potential to provide two or more the electrical current of Li-ions (depending on the charge of the multivalent ion) if present with the same density. Calcium ion and magnesium ion provide relatively high potentials (Ca2+: −2.87 relative to standard hydrogen electrode, SHE; Mg2+: −2.37 vs. SHE) suitable for many secondary battery requirements. One of the most relevant aspects is understanding the mechanism of reversible cycling of Mg during battery operation and recharging. So far, reversible Mg plating has been achieved with only a narrow class of electrolytes, inorganic or organic magnesium aluminum chloride salts dissolved in ethereal solutions. For example, the Mg analogues to the most common commercial Li-ion electrolytes instantaneously decompose and passivate the Mg metal anode surface preventing further electrochemical reaction, consequently blocking the battery.
Interest in advanced secondary magnesium ion batteries blossomed with the introduction of the Mg—Mo6S8 battery. In recent years, in contrast to the rapid progress of effective electrolytes capable of plating/stripping magnesium reversibly with wide electrochemical windows, the development of efficient cathode material is far behind. Materials such as MnO2, V2O5, and even novel layered oxyfluoride have been reported to facilitate Mg2+ ion intercalation, however, they also have problems such as irreversible intercalation and low capacity. To date, Chevrel phase Mo6S8 is still the most reliable cathode material with long-term cycling performance in rechargeable magnesium ion batteries. However, Mo6S8 only has a theoretical capacity of around 130 mAh/g, and can only operate at less than 1.3 V (vs Mg), which is not desirable for high density energy storage. Thus, the pursuit of higher voltage and higher capacity cathode material deserves great attention for magnesium ion batteries nowadays. Unfortunately, the traditional transition metal oxide-based cathode materials for lithium ion batteries show intercalation difficulty of the divalent and high charge density Mg2+ ions. Redox active organic material is another type of promising alternative for magnesium ion batteries, especially in terms of the resource sustainability and environmental friendliness. Surprisingly, although great success has been achieved for organic cathodes in lithium and sodium ion batteries, only a handful of organic cathodes have been reported for magnesium ion batteries in the literature. A common issue for the organic cathodes is that either they can only deliver low capacity even at very slow current rates, or they suffer from considerable capacity loss upon cycling.
here are ongoing needs for new conductive polymers for multivalent ion batteries, particularly Mg-ion batteries, and for improved methods of preparing such polymers. The polymers and methods described herein address these needs.